Although this invention is applicable to the dispensing of various liquids, it has been found particularly useful in the environment of dispensing very small solder balls to very small soldering areas. Therefore, without limiting the applicability of the invention to "dispensing very small solder balls to very small soldering areas", the invention will be described in such environment.
In a high density electronic manufacturing process, semiconductor integrated circuit chips are bonded to a substrate by a solder reflow process. In the prior art and with reference to FIG. 1, the semiconductor integrated circuit chip or chip 10 is shown operatively joined to substrate 12 after the solder reflow process has been completed.
Also in the prior art and with reference to FIG. 2, the interconnect material between the chip 10 and the substrate 12 is in the form of a solder bump 14 prior to the solder reflow process. The solder bumps 14 are placed on the pads 16 of the integrated circuit chip or chip 10 by one of various prior art processes which include 1) deposition through a mask plus solder reflow, 2) electroplating plus solder reflow, 3) pick-and-place of $ solder bumps, etc. The actual bonding process utilizes solder bumps which are deposited on the wettable metal pads 16 on the chip 10. Chip 10 (the upside-down chip or flip chip) is then flipped over (see FIG. 2) and the solder bumps 14 are aligned with the correct matching o footprint of solder wettable terminals or connect pads or bond pads 18 on the substrate 12. Heat is then applied and all joints or interconnects between the pads 16 on the chip 10 and the solder wettable terminals or connect pads or bond pads 18 on the substrate 12 are made simultaneously by reflowing the solder in the solder bumps 14. Typically, the interconnects are made with solder bumps 14 which are 100 to 150 .mu.m (microns) in diameter with several hundred to several thousand solder bumps per chip. The electronic manufacturing process is moving toward more interconnects per chip which requires solder bumps of a smaller size since the interconnects are closer together.
With reference to FIGS. 3 and 4, the prior art method of deposition of solder through a mask plus reflow for creating solder bumps 14 is disclosed. Solder is evaporated through apertures or holes 20 in a metal mask 22 which has been aligned over the metal pads 16 on chip 10 with the metal pads 16 being surrounded by insulation 24. In this way an array of solder deposits 26 comprising tin 28 and lead 30 is created on the surface of the chip 10. This method is a batch process. The composition of the evaporated solder deposits 26 depends upon the starting composition of the solder and the vapor pressures of the respective metals comprising the solder. Reflow in a furnace homogenizes the solder deposits 26 and brings the solder deposits 26 to a spherical shape or solder bumps 14 as shown in FIG. 4. The metal pads 16 are wettable by the solder bumps 14 while the insulation 24 is not. Some disadvantages of this process include the high overall cost of the process. The quality and repeatability of the solder alloy is not good and the masks get coated with the solder materials and must be continuously cleaned or thrown away.
Another process for creating solder bumps 14 is the electroplating process plus reflow and consists of application of photoresist material on the chip, exposure to a masked ultraviolet beam to define a pattern in the photoresist material, development of the pattern in the photoresist material and then electroplating with the solder composition. Reflow in a furnace brings the solder deposits to a spherical shape similar to that in FIG. 4. Some disadvantages of this process include the high overall cost of the process because of the photolithograph and the solder plating step presents environmental waste treatment problems. Furthermore the solder alloy is not consistent and usually contains impurities.
Another process for creating solder bumps 14 is the pick-and-place method wherein solder balls are created off line using processes developed for the creation of lead shot. The solder balls are then inspected and those which do not meet specifications are discarded. The solder balls are arranged in grooves in a tray and a robot arm generates the pick-and-place action and picks up solder balls, one at a time and, with the application of heat and pressure, positions each solder bump on an individual pad 16 on chip 10. Some disadvantages of this process include the high cost of the equipment needed for the process, the fact that the process is not suitable for volume production and the danger to individuals in handling and controlling the small solder balls.